A novel topoisomerase (topoisomerase V) from a thermophilic archaebacterium has been shown to have properties similar to eukaryotic topoisomerases and quite distinct from other bacterial enzymes of this class. It relaxes either negatively or positively supercoiled DNA in a reaction not dependent on a divalent metal ion, attaches to the 3' end of the DNA chain, and shows a striking cross-reactivity with antibody against human topoisomerase I. An evolutionary relationship between archaebacteria and eukaryotes has often been discussed; the properties of this enzyme support the idea. In further studies of DNA gyrase, mutations of the ATP binding site have been used to show that both B subunits in the active A2B2 complex must be capable of hydrolyzing ATP in order for the enzyme to catalyze DNA supercoiling. Transcription from E. coli promoters that are stimulated by DNA relaxation is also able to read through termination signals some distance away, and this readthrough is further increased by DNA relaxation. This coupling between events at separated sites is evidence for the use of other factors beside RNA polymerase. An assay for such factors has been devised and is now being used in a purification scheme.